Triggered by the erudite debate in Ampdog's thread Abnormal pentode connection, I have a question at a far lower level.
In the circuit diagram for the Leak TL/25A amp (shown below) referred to in that thread, a KT61 tetrode drives the screen grids of the KT66 power valves. My guess is this is to keep the screens at about 2/3 of B+ (~300V). How does this arrangement work exactly? I'm trying to work out what the quiescent/steady-state condition would be (a screen current of about 1/10 to 1/5 of plate current?) and how it is maintained, then what happens when there's a signal. What are the advantages compared to say just a resistor calculated to provide the same quiescent screen voltage and/or current? Also, why is the heater tied to the cathode - noise reduction?
In the circuit diagram for the Leak TL/25A amp (shown below) referred to in that thread, a KT61 tetrode drives the screen grids of the KT66 power valves. My guess is this is to keep the screens at about 2/3 of B+ (~300V). How does this arrangement work exactly? I'm trying to work out what the quiescent/steady-state condition would be (a screen current of about 1/10 to 1/5 of plate current?) and how it is maintained, then what happens when there's a signal. What are the advantages compared to say just a resistor calculated to provide the same quiescent screen voltage and/or current? Also, why is the heater tied to the cathode - noise reduction?
